From what I gather, Qt has "won" the war, if there was ever such a thing, but GTK and Cairo will not be going away. What are your gripes with Cairo, anyways?

Also, when you start getting into scientific software, very few projects explicitly state "Runs best on Linux" even though that may be the case. If you are looking for examples though, I will give some: PETSc/Elemental/Trilinos/etc Clawpack/PyClaw/OpenFOAM/etc Gromacs/NAMD/HOOMD/Quantum Espresso/etc Julia/Nimrod/etc Paraview/VisIt/VMD/etc OpenMPI/MPICH/MVAPICH/etc ImageJ/Fiji/etc GNU Radio/RTL-SDR/etc

There's tons of it out there!